Ozone Research- Past—Present—Future'

Ozone Research- H. U. Diitsch Atmospheric Physics, ETH Past—Present—Future' 8093 Zurich, Switzerland

The 1970s brought us a rapid expansion in ozone research stratospheric circulation, which was then only rather vaguely showing the importance of that particular trace gas to man- known. kind in a new light. It seems, therefore, at the beginning of a These findings led (largely in connection with the IGY [In- new decade, appropriate to look back to the origin and to ternational Geophysical Year]) to another effort in the ob- consider recent developments in the context of the early ef- servational field. A strong expansion in the spectrophotome- forts in ozone research. This may help in the planning for the ter network (total ozone), including reliable measurements immediate future and for the solution of more extended during the polar night, was followed by the development of long-range problems. instruments and techniques for the operational measurement of the vertical ozone distribution: optical sonde, Kulcke and Paetzold (1957); electrochemical sonde, Brewer and Milford 1. Short history of ozone research (1960); chemiluminescent sonde, Regener (1960); and standard computer-evaluation of Umkehr data, Mateer and It is now almost 100 years since the discovery of the strong Diitsch (1964). First in conflict, but later in agreement, with UV-absorption of ozone by Hartley suggested that this gas indications from radioactive debris data, it was inferred from was responsible for the cutoff of the solar spectrum near these new observations that ozone transport resulted from 300 nm, and some 60 years have passed since Fabry and Buis- the combined action of meridional cells and large-scale son proved this beyond doubt and provided at the same time exchange. a convenient method for measuring the total ozone content At the same time, however, some discrepancies between overhead—thus really getting research on atmospheric the accumulating new observational material and the classi- ozone started (after some scattered preceding efforts). cal (Chapman) photochemical theory evolved, which could It is, in the view of the present development, noteworthy not or only with great difficulties be explained by any kind of that the interest in atmospheric ozone had initially two seem- transport (incorrect vertical gradient in the upper part of the ingly unrelated roots. One (represented by Dobson) was layer, differences in global ozone burden, etc.). Thus ozone connected with weather (it was then thought that tropo- researchers were receptive to Hampson's (1964) suggestion

spheric cyclones might be produced by excess heating of the of the importance of HOx production by O('D) originating stratosphere by higher ozone content over some regions); from ozone-photodissociation (by wavelength below about Gotz, on the other hand, became interested in ozone because 310 nm), an idea which was further pursued by Hunt (1966) he studied UV-B radiation, which is a function of the atmos- and others. pheric ozone content, in connection with its medical value By this the way was broken for the introduction of other (see Fig. 1). trace substances into the photochemical theory of ozone. Both interests joined in building the first small ozone net- Crutzen (1970), Johnston (1971), and Nicolet (1971) showed work, which provided already in the 1920s the basic knowl- the importance of NO*, which is also produced in the strat- ! edge on ozone distribution. It was soon followed by Chap- osphere by 0( D) (from N20, which is of tropospheric origin man's (1930) theoretical explanation of the ozone layer by his like the H20). Finally, after Stolarski and Cicerone (1974) photochemical theory, which gave results in reasonably good had suggested CIO* as providing a further catalytic cycle for agreement with the first measurements of the vertical ozone ozone destruction, Molina and Rowland (1974) pointed out distribution (Gotz et al., 1934; Regener and Regener, 1934). that CFMs [chlorofluoromethanes] would be a major and Further development and application of the theory during increasing source for these radicals. the 1940s (Diitsch, 1946; Schroer, 1949; Craig, 1950) showed The 1970s thus showed ozone to be the central substance that the observed ozone distribution could only be under- of a complicated stratospheric trace gas system, which in stood if it was strongly influenced by meridional ozone turn controls its concentration. This introduced the possibil- transport by the stratospheric circulation during the cold ity of anthropogenic influences on the ozone layer. It also season; the high photochemical relaxation times calculated provided new physical mechanisms as links between solar ac- for the lower part of the ozone layer supported such an ex- tivity and atmospheric ozone. Possible consequences of strat- planation. It became clear that ozone was a useful tracer for ospheric pollution by human activities had now to be considered. In order to do this it was necessary to try and get a quanti- 1 Presidential address given at the Quadrennial Symposium on At- tative grasp of the ozone destruction efficiency of the various mospheric Ozone, 4-9 August 1980, Boulder, Colo., a symposium of the International Ozone Commission of the International Associa- radical groups, considering also interactions between them. tion of Meteorology and Atmospheric Physics, cosponsored by First of all, a concerted effort in laboratory kinetics was WMO, AMS, COSPAR, and NASA. needed to determine a wealth of rate constants. This had to 0003-0007/81/020213-05$05.25 be supported by an as accurate as possible determination of ©1981 American Meteorological Society all kinds of absorption coefficients, plus their combination

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FIG. 1. The development of ozone research since 1920. with redeterminations of the solar spectrum in order to pro- full grasp of the problem. This was required on one hand to vide reliable dissociation rates. On this basis chemical model- get a reasonably safe basis for the parameterization of trans- ling became possible. port in the models and on the other side for checking whether Already the classical theory had to coexist with strato- the chemical modelling was at all reasonable. spheric ozone transport—but chemistry and transport could That this was not self-evident was clearly shown by the fact be relatively easily combined or, if necessary, separated. In that during the 1970s the prediction of the effects of a large the modern theory the two have become, for most purposes, SST-fleet on the ozone layer changed from a considerable re- thoroughly interwoven because not only the ozone but also duction to a slight increase as our knowledge of the rate con- the distribution of the trace gases is strongly influenced by stants improved and also due to the successive completion of circulation. Most of the parent substances of radical groups the reaction system, especially with respect to the importance originate from the earth's surface but become only active in of the interaction between the different radical groups. For the stratosphere. Therefore 1-, 2-, or 3-dimensional model- similar reasons the predictions on the expected CFM-influ- ling of transport had to be combined with the chemical ence on the ozone layer have also undergone considerable modelling. fluctuations. As the high natural variability of the atmos- Ozone observations had now to be supplemented by meas- pheric ozone content does today not allow us to prove or dis- urements of as many pertinent trace gases as possible with prove directly any anthropogenic influences on the layer, ob- concentrations partly in the ppt-range in order to obtain a servational evidence about the trace gas system that modern

Unauthenticated | Downloaded 10/05/21 03:49 PM UTC Bulletin American Meteorological Society 215 theory has built around the ozone is the only way to substan- mixing parameterization in 2-dimensional models has been tiate whether the modern photochemistry is at all reasonable, strongly questioned on the basis of 3-dimensional experi- and if yes, whether the deductions made from it are more or ments. A different approach has recently been proposed less correct. (Pyle, 1979); however, its success has yet to be proved. It This seems largely to be the case, although some inconsis- seems that it would be most profitable to make a strong effort tencies still remain. It thus became necessary to look into the in this direction unless it became feasible to put a reasonably possible consequences of anthropogenically introduced complete chemistry into a General Circulation (3-dimen- changes in the ozone layer. Interestingly enough this brings sional) Model. us back to the problems that led to the instigation of ozone research 60 years ago: On the one hand possible influences on climate must be considered that could be produced by changes in the radiation balance at the earth surface as well b. Observations as through the expected changes in stratospheric dynamics, whilst on the other hand UV-B radiation reaching the earth's Further experiments on large stratospheric balloons as they surface would be altered with possible biological consequen- were successfully performed recently will be of prime impor- ces. These two problems are, in addition, not independent of tance for checking on the chemical part of the models (items each other; changes in climate will certainly influence life, 1 and 2 in the previous paragraph), i.e., experiments to meas- whilst UV-influence on marine life could alter the albedo and ure, in addition to ozone, as many of the pertinent radicals as thus climate; such changes in the surface layer of the ocean possible, simultaneously and in situ. If these measurements

could also provide a link with the C02 problem. Further could follow a fixed air parcel while also obtaining a measure there is a feedback from both sides to the ozone system, of 02 photodissociation, a full determination of the instan- through changing transport and through the biological in- taneous odd oxygen budget could be made (first steps in this fluence on the trace gas system. Undoubtedly ozone research direction have already been taken by Anderson (1979)). These has become strongly interdisciplinary. types of measurements must be supported by global observa- This is the point that we have reached now and from where tions of the long-lived trace species (CFM, N20, CH4, H20, we can visualize a number of problems that must be tackled and possibly HNO3 by direct (balloon-borne) and remote immediately, i.e., during the next few years; and some others sensing techniques (mainly by satellites)). These measure- that undoubtedly will require more time for a solution—of ments should provide the necessary information for checking the order of a decade or more. This does not, however, mean on the transport part of the modelling. that work on this latter group should not also be started right Efforts towards an improved ozone climatology will still now. play an important role in this context. Especially the geo- graphical and seasonal variations of the vertical ozone distribution including year-to-year changes are not yet well 2. Immediate future enough known. A combination of ozone soundings (especially for establishing long-term series) and remote sensing It was shown in the previous section (see Fig. 1) that theory techniques from satellites, but possibly also from the ground, and observations first advanced in an alternating mode and will be needed. Such data are important as a base for photo- became more thoroughly interconnected during the last 10 chemical budget calculations yielding ozone flux divergences years. It will be of utmost importance that such rapid interac- as a tool for improving the 2-dimensional parameterization tion be further improved in the future. of transport. In this connection the global ozone budget should be studied with increased emphasis on its tropospheric part, including further experiments on the flux through the tropopause and on the destruction at the surface a. Theory (topography with its related local circulations seems to be of some importance). The photochemistry in this source region Photochemical modelling needs improved input from three of most long-lived stratospheric trace gases is of particular sides: 1) Concerted efforts have to be made to lead the reac- relevance. tion scheme to the best possible completeness—although it The correctness of the reaction system used in the photo- must be realized that this goal will probably never be chemical models should in addition be studied by specific achieved fully. 2) Regardless of the recent advances in labor- types of measurements (for example the ratio of HF/HC1). atory work, the reliability of rate constants should be further As this is a rather lengthy list, considerable thought (by improved, as the largest quantifiable uncertainty is still re- cooperation between modellers and experimentalists) must lated to them. In parallel, some dissociation rate values yet go into an optimization of the observational effort. should also be redetermined by combining new absorption Finally, for estimating the anthropogenic impact on some measurements with solar flux information—whereby the trace gas concentration and thus its influence on the ozone possible variability of dissociation rates during the solar layer, it is necessary to obtain an improved understanding of cycle should also be studied. 3) Perhaps the most open prob- some trace gas cycles (natural or other), e.g., for N20, but lem is the modelling of transport. One-dimensional models, also CFMs; some of them are intimately tied to more general so far mostly used in predictive studies, can hardly be further cycles like that of nitrogen. Suitable observations, as well as improved; however, it is well known that they can never give particularly aimed experiments, together with theoretical the full answer. The validity of the generally used large-scale consideration will lead to that goal.

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3. Extended projects tackle the problem with a General Circulation Model including full chemistry and also to study by this means the possible While the immediate work will be concentrated on a full un- influence of solar variability, possible albedo changes (due to derstanding of the ozone-trace gas system and thus on ob- the biological impact), and the coupling with the CO2 prob- taining reliable forecasts of anthropogenic influences, the eval- lem—especially its expected influence on stratospheric uation of possible consequences of such changes will need temperature. This seems hardly possible with the present continued efforts over many years—over a decade or more— computer capability. Conceivable are, however, experiments and many of the related questions can only be answered on like the following: Full chemistry in the ozone-trace gas the basis of the research described in the previous section. model combined with a 2-dimensional model of strato- Two main consequences are visualized presently as al- spheric circulation, including improved parameterization; ready indicated above. They are not unrelated and both of full General Circulation Model with emphasis on good tro- them are to some extent coupled with man's other major im- popause representation with parameterized chemistry; or a pact on the atmosphere, which is rapidly growing at present, General Circulation Model with simple chemistry and a full namely, the C02 increase by fossil fuel burning. chemical model without transport used iteratively together. This theoretical effort must go in parallel with an observational system of high quality, long-range stability—i.e., by a. Influence on climate monitoring the worldwide distribution of total ozone and its vertical distribution with the special aim to separate long- Solving this problem means to come to a full understanding term trends from the effects of sudden warmings, of biennial of a complicated feedback system that is possibly also influ- variation, and of solar activity. Most long-lived trace gases as enced by solar activity in a rather involved way (Fig. 2). The N20, CFMs, CH4, H2O, H2, and eventually total NO, and influence of a changing ozone layer on climate has two roots: ClO^, must be included in such a monitoring program. U V-solar radiation and the earth's albedo are other parame- 1) The ozone change and the changes in the related trace ters involved. This observational system must grow out of gas system may directly influence global temperature that described in the previous section, whereby a certain re- by altering the radiation balance at the surface. At laxation should become possible on the basis of increased present such effects are mostly thought to be small be- understanding of the whole problem. Monitoring is also cause some of the terms are of opposite sign; this result needed for continuous verification of the forecasts made by may, however, not be final. the models already existing or developed in the near future. 2) A change in the ozone layer will undoubtedly lead to considerable alterations in the stratospheric temperature field, especially above about 30 km, and by this to b. Biological consequences a change in stratospheric circulation. While the strong influence of the troposphere on the stratospheric circu- The solution of this problem needs intensive cooperation lation is reasonably well understood, the feedback among biologists, research workers in the ozone field, and from above is the big unknown in this whole problem. radiation specialists. While presently skin cancer and similar While some scientists ^ssume it to be practically nil, possible influences on man are most intensively discussed which makes the ozone-climate problem look rather (whereby it is questionable whether they would all be nega- unimpressive, others feel that there is a definite strato- tive), adverse influences on well-adapted ecosystems might in spheric influence onto the troposphere, largely in con- the long run prove to be more important. Presumably em- nection with the development in the lower layer during phasis must be put on the effects on important cultivated sudden warming events. A possible influence of strat- plants (food production) and on the influence on marine life ospheric changes on the tropospheric circulation (algae etc.), which is the basis of long food chains and which through variations in the tropopause level is also may lead to changes in albedo and in the C0 uptake by the conceivable. 2 oceans, thus having implications on the climate problem. All On the theoretical side, it would certainly be desirable to such studies must carefully consider the season- and latitude- dependence of total ozone and the related variations of UV-B. Full reward from the proposed research will only come along if modelling and observations are interconnected as intimately as possible and if this research becomes really interdisciplinary.

References

Anderson, J., 1979: Free radicals in the earth atmosphere. Rept. FAA-EE-80-20, NATO Advanced Study Institute on Atmospheric Ozone. FIG. 2. The ozone-climate feedback system under the influence of Brewer, A. W., and J. R. Milford, 1960: The Oxford-Kew sonde. solar variability and fossil fuel CO2. Proc. Roy. Soc. London, A256, 470-496.

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Chapman, S., 1930: A theory of upper atmospheric ozone. Phil. Kulcke, W., and H. K. Paetzold, 1957: Ueber eine Radiosonde zur Mag., 10, 345-352. Bestimmung der vertikalen Ozonverteilung. Ann. Meteorol., 10, Craig, R. A., 1950: The observations and photochemistry of atmos- 63-75. pheric ozone and their meteorological significance. In The Obser- Mateer, C. L., and H. U. Diitsch, 1964: Uniform evaluation of vations and Photochemistry of Atmospheric Ozone, Meteorol. "Umkehr" observations from the world network. I. Proposed Monogr., 1 (2), AMS, Boston, 50 pp. standard Umkehr evaluation technique. NCAR, Boulder, Colo. Molina, M. J., and F. S. Rowland, 1974: Stratospheric sink for chlo- Crutzen, P. J., 1970: The influence of nitrogen oxides on the atmos- rofluoromethanes: chlorine atom catalysed destruction of ozone. pheric ozone content. Quart. J. Roy. Meteorol. Soc., 96, 320-325. Nature, 249, 810-812. Diitsch, H. U., 1946: Photochemische Theorie des atmospharischen Nicolet, M., 1971: Aeronomica acta A 79, 1970. In Mesospheric Ozons unter Beriicksichtigung von Nichtgleichgewichtszustanden Models and Related Experiments, D. Reidel, Dordrecht, pp. 1-51. und Luftbewegungen. Thesis, Univ. of Zurich, 113 pp. Pyle, J. A., and C. F. Rogers, 1979: Transport processes. Rept. FAA- Gotz, F. W. P., A. R. Meetham, and G. M. B. Dobson, 1934: The EE-80-20, NATO Advanced Study Institute on Atmospheric vertical distribution of ozone in the atmosphere. Proc. Roy. Soc., Ozone. A145, 416-446. Regener, E., and V. H. Regener, 1934: Aufnahme des ultra-violetten Hampson, H. J., 1964: Photochemical behavior of the ozone layer. Sonnenspektrums und vertikale Ozonverteilung. Phys. 2., 35, 788. Tech. Note 927, Can. Arm. Res. Develop. Establ., Valcartier, Regener, V. H., 1960: On a sensitive method for the recording of at- Quebec, 280 pp. mospheric ozone. J. Geophys. Res., 65, 3975-3977. Hunt, B. G., 1966: The need for a modified photochemical theory of Schroer, E., 1949: Theorie der Entstehung, Zersetzung und Vertei- the ozone layer. J. Atmos. Sci., 23, 88-95. lung des atmospharischen Ozons. Ber. Deut. Wetterdienstes (US- Johnston, H. S., 1971: Reduction of stratospheric ozone by nitrogen Zone), 11, 13-23. oxide catalists from supersonic transport exhaust.Science, 173, Stolarski, R. S., and R. J. Cicerone, 1974: Stratospheric chlorine: a 517-522. possible sink for ozone. Can. J. Chem., 52, 1610-1615. •

announcements (continued from page 212) vided they have not been submitted for publication elsewhere. Com- pleted typescripts should be brought to the symposium forconsider- ation by the program committee, where they will be refereed before being accepted for publication. Symposium on Global Water Budget For complete information on the symposium contact: R. E. A symposium to investigate the physical processes that control sea- Newell, Program Committee Chairman, Department of Meteor- sonal and nonseasonal variations of the global water budget will be ology, 54-1520, Massachusetts Institute of Technology, Cambridge, held at the School of Geography, Oxford, England, during 9-15 Mass. 02139 (tel: 617-253-2940). August 1981. Entitled "Symposium on Variations in the Global Water Budget," the meeting is sponsored jointly by the International Commission on Climate of the International Association of Meteor- ology and Atmospheric Physics, the International Association of Hydrological Sciences, the Paleoclimate Commission of the Interna- Symposium on Scientific Results from Seasat— tional Union for Quaternary Research, and the Joint Scientific Call for papers Committee of the World Meteorological Organization/Interna- tional Council of Scientific Unions' World Climate Research The Symposium on Scientific Results from Seasat is being sponsored Program. jointly by The American Geophysical Union (AGU), the Seasat A major aim of the symposium is to increase understanding of the Data Utilization Project at the Jet Propulsion Laboratory, and the modes of variation of the water budget over various regions of the Office of Space and Terrestrial Applications at the National Atmos- globe and their relationship to changes in the large-scale circulation pheric and Space Administration Headquarters. It will be an integral of the atmosphere and oceans. The sessions will present measure- part of the 1981 Spring Meeting of AGU, to be held in Baltimore, ments taken over various time scales, and the scientific program will Md., during 25-29 May 1981. The Symposium will present reports on cover the following topics: overview of seasonal variations; recent scientific investigations that utilize Seasat data in the disciplines of fluctuations in middle latitudes: rainfall, drought, river flow, meteor- oceanography, meteorology, geodesy, and glaciology. Abstracts, in ological causes; recent fluctuations in tropical latitudes: Africa, standard AGU format, should be submitted according to AGU South America, monsoon regions of the Eastern Hemisphere, and procedures and deadlines. Information on procedures and deadlines maritime regions; rainfall over the oceans: comparisons of island may be obtained from, and abstracts should be sent to: American data with satellite and ship data; measurements of atmospheric water Geophysical Union, Spring Meeting, 2000 Florida Ave., Northwest, content by satellites, aircraft, special-purpose balloons, and radio- Washington, D.C. 20009. sondes: comparisons, variations, and interpretations; paleoprecipi- A special issue of the AGU Journal of Geophysical Research tation patterns: pollen, lake-level, and aeolian evidence; fluctuations (JGR) will be devoted to scientific results from Seasat in the areas in high latitudes and high altitudes: isotopes in ice cores, changes in cited above. Papers presented at the Symposium, and others, will be permafrost, glaciers, sea ice, and snow lines; spatial variations in the welcomed for consideration. Papers submitted for this issue will be isotopic composition of precipitation and groundwater and their reviewed by standard JGR procedures. Details and schedules for relationship to atmospheric circulation; the ocean water balance and manuscript submittal will be announced at the Spring AGU meet- its long-term fluctuations; and global synthesis of water-cycle ing. In order to assure timely publication, papers are expected to be changes. submitted shortly after the Symposium. Participation will be restricted to —150 people to encourage dis- cussion. It is hoped that a special volume will be published after the meeting that will include papers presented at the symposium, pro- (icontinued on page 225)

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